A gear that is used in, for example, a transmission for an automobile is required to have high pitting resistance and bending fatigue strength. In order to meet such a requirement, a carburizing treatment has conventionally been widely performed as a method of strengthening a steel member such as a gear. Further, with the aim of further improving the pitting resistance, there has been proposed an invention relating to achievement of high strength by a carbonitriding treatment (Patent Document 1). On the other hand, in a planetary gear, due to its high engagement degree, tooth profile accuracy (strain) has a great influence on gear noise. Especially in an internal gear, there has been a problem that it is likely to be strained because it is thin and large in diameter. Under such circumstances, there has also been proposed an invention relating to a gas nitrocarburizing treatment which causes a steel member to strain little and also causes only a small strain variation (Patent Document 2).